The present invention relates to X-ray diagnostic imaging systems and more particularly to devices which regulate the current flowing through the filament of the X-ray tube in such systems.
A conventional X-ray system has a vacuum tube which when electrically excited emits X-rays. The tube includes a filament to heat a cathode in the tube to an operating temperature. Once at this temperature, a high DC voltage is applied across the cathode and an anode resulting in an electron beam bombarding the anode to produce X-ray emission. The X-ray tube can be electrically modeled as a variable resistor, the resistance of which being a function of the temperature of the tube's filament, and therefore the filament current. Within the filament's operating temperature range, the emission current flowing between the anode and the cathode, and hence the X-ray emission, is proportional to the filament current.
Because of the hazards associated with overexposure to X-rays, as well as the need to control the exposure for accurate imaging, it is necessary to closely regulate the X-ray emission. One previous method of accomplishing this regulation, disclosed in U.S. Pat. No. 4,930,145, continuously compared the actual anode-to-cathode voltage to a reference level and varied a filament current power supply, based on the result of the comparison, until the desired anode-to-cathode voltage was achieved. The filament current also was sensed to produce a feedback signal which controlled the filament current power supply.
Typically a regulated DC source was used as the filament current power supply since it is easier to regulate to a constant level than the RMS output of an AC supply. The DC output from the source was coupled to a chopper circuit whose load was the primary winding of a transformer with the tube filament connected to the transformer secondary winding. Low losses where achieved by switching the power supply with fast rise and fall times to minimize the heat dissipated in the switching device. In view of the rapid switching of current to the transformer, a resonating capacitor was connected across the primary winding.
As the filament age, its electrical resistance tends to increased and this change was coupled through the transformer into the power supply. The change in resistance altered the Q of the resonant transformer circuit and thus the magnitude of the resonant circulating current. Because the resonant circulating current was inductively coupled into the secondary winding and the filament, such variation of the resonant circulating current changed the total current through the filament in an unregulated manner.